Rotary tools of the drill or boring tool type usually use chucks to hold drill bits or boring tool bits. Many types of such tools--hereinafter for short "drills"--permit adjustment of the operating speed of a drive motor within wide limits without changing gearing in the motor. The speed which is to be selected at which a drill bit or boring bit is to operate depends on the diameter of the hole to be made or reamed, the material in which the hole is being made, and the quality of the drill or boring bit. Other types of such machine tools have internal gearing, which can be selected as desired, to provide higher torque, or higher speeds, respectively, at the tool bit with a given speed of the drive motor.
The speed of operation of the tool bit is usually determined either by experience by the operator, or taken from a table. Frequently, it is merely estimated. Tables which are available for preferred speeds, considering the diameter of the hole to be worked, the materials and the particular tool bits, are often ignored, particularly if there is time pressure on the operator. Estimating the speed within the wide operating range which electrical drills are capable of is frequently coupled with uncertainty. Less than highly skilled or experienced operators often operate drill bits, boring bits and the like at improper speeds. The result is that, either, the tool bit is damaged by too fast rotation or feed, so that it will heat excessively and may even become incandescent; or the speed of operation as well as the resulting hole may be improperly made.
The referenced Swedish Patent Disclosure Document SE-OS No. 78 07 412-7 describes control systems in electric boring machines in which the motor speed is controlled as a function of the tool bit diameter secured in the chuck of a drill or boring machine. As described, transmission of the data representing diameter of the drill bit to the control system is carried out mechanically. The actual construction is complex to build, and it is difficult to obtain a suitable transmission path from the rotating chuck to the control system which is fixedly secured to the housing or frame or structure of the boring machine. The signals which are derived are not dependent on speed, only on diameter of the tool bit. The control system, thus, is controlled only in relation to the diameter of the tool bit. Matching the speed of the operation of the tool bit to different materials which are to be drilled or bored is not possible in the system as described.
Other types of speed control systems are known, in which the speed of rotation of the tool bit is controlled, the speed itself being derived from the rotary speed of the motor itself. The controller then determines the operating speed of the drive motor. If the drill is a multi-geared drill, then the speed of the tool bit itself additionally depends on the transmission ratio of the interposed gearing. Thus, the control of the speed of operation of the tool bit can be adjusted in two ways: by controlling the speed of the motor, and by setting the transmission ratio of the drive transmission. It is, thus, possible to obtain a single speed of the tool bit in two different ways. The possibility of obtaining this single speed in two ways further increases the difficulty in finding the proper speed for the tool bit and also interferes with easy readability of speed tables which relate tool bit speeds to different materials. For example, a higher drive torque is obtainable by selecting a certain transmission ratio; this, then, requires readjustment of the speed controller in order to obtain the same drive speed which may have been selected previously at a different transmission ratio. This is particularly annoying if switch-over of the gearing was done only in order to match the load resistance placed by the material on the drill to the drive torque of the motor.
The operating range of the drill is extended if a combination of variable speed motor and shiftable transmission gearing is used in the drill. This drill can then easily be used with drills of different diameters for various types of materials by following suitable tables. From experience, however, it has been found that the proper association of speed and gearing is frequently ignored by operators.